Providing essential oils and/or saponins such as, garlic oil, cinnamon bark oil or powder, and lemongrass oil to an environmental medium to disrupt the enzyme and coenzyme systems, that are integral parts of the methanogenesis process. The disruption limits the growth and reproduction of the methanogens that normally compete with slower-growing, halo-respiring bacteria for the hydrogen donors within the environmental medium (either naturally occurring or provided via fermentable substrates provided to the environmental medium of part of a reduction process). The amount of methane produced during in situ remediation processes is also reduced. The essential oils/saponins are harmless to the slower-growing, halo-respiring bacteria that are utilized to dechlorinate containments in the environmental medium. The essential oils/saponins can be provided alone or along with various organic hydrogen donors, zero-valent iron (ZVI) or other surfactants in order to enhance the biodegradation (reductive dechlorination) of the targeted contaminants.